Ultra-low noise fiber laser systems and their applications

Fiber laser systems are intensely studied for and already utilized in a wide range of scientific, biomedical and industrial applications. Scientifically, fiber lasers are widely used for spectroscopy, laser-matter interactions, nonlinear and quantum optics experiments, among others. The industrial applications range from the well-established, such as laser-material processing, laser marking, and various forms of optical sensing to niche or upcoming applications such as highspeed circuit testing, inspection of packaged foods, additive manufacturing. In all applications outside the research laboratory, long-term stability of the lasers operation is of paramount importance. Fiber lasers are clearly advantageous in this respect, as the optical fibers provide isolated paths for light propagation, minimizing the impact of environmental effects, and generally render the laser system nearly or completely free from mechanical misalignment. In addition to long-term stability of the laser operation, short-term (typically less than 1 second) stability, or fluctuations of the laser output is of crucial importance as in many situations, it effectively determines the signal-to-noise ratio, sets the resolution or otherwise limits the quality of the measurement. Fluctuations or noise impact both the intensity and phase of the laser output. As part of this thesis, first, the intensity noise of mode-locked fiber lasers is characterized systematically for the major mode-locking regimes over a wide range of parameters. It is found that equally low-noise performance can be obtained in all regimes. Losses in the cavity influence noise strongly without a clear trace in the pulse characteristics. Noise level is found to be virtually independent of pulse energy below a threshold for the onset of nonlinearly induced instabilities. Instabilities that occur at high pulse energies are characterized. It is found that continuous-wave peak formation and multiple pulsing influence noise performance moderately. However, at high pulse energies, an abrupt increase of the intensity noise is encountered, corresponding to up to 2 orders of magnitude increase in noise. These results effectively constitute guidelines for minimization of the laser noise in mode-locked fiber lasers. For the high-power laser systems that utilize external amplification in fiber amplifiers, the added noise due to amplification is usually predominantly determined by the pump source, assuming that the amplifier design is correctly made and amplified spontaneous emission (ASE) is minimized. Many high-power amplifiers utilized multi-mode pump diodes, which have much higher noise levels. A high-power fiber laser system where the amplifiers are seeded by low intensity noise pulses is analyzed in detail. When operating at its maximum power level (10 W), the amplified output exhibits an integrated (from 3 Hz to 250 kHz) intensity noise of 0.2%, whereas the seed signals intensity noise is less than 0.03%. The origins of the added noise is analyzed systematically using modulation transfer functions to ascertain contributions of the pump source. The transfer of the noise in the seed signal is also analyzed, as well as contributions of ASE, which can be significant. Prediction of intensity noise by modulation transfer functions supplies a lower limit for the intensity noise of fiber lasers and amplifiers. The second part of the thesis applies the know-how on low-noise fiber lasers that was developed in the first part to a scientific problem. As part of a collaboration with researchers from Ruhr-University at Bochum, Germany, we have developed a custom, low-noise laser system for spectroscopy of micro-plasma discharges. Absorption spectroscopy is a commonly used technique to determine the presence of a particular substance or to quantify the amount of substance present in the plasma discharge. However, the absorbance is usually small, at the level of one part in a thousand or less. Therefore, low-noise laser signals are required to detect such low differences. We developed a low-noise fiber laser system for the absorption spectroscopy studies of reactive species in a micro-plasma discharge. The laser setup also produces high-energy picosecond pulses, which are powerful enough to trigger the plasma ignition and transition into other transient states of plasma. Since both pulses are generated from the same mode-locked oscillator, they have excellent mutual synchronization. We demonstrate the possibility for pump-probe experiments by initiating breakdown on a picosecond time scale (pump) with a high-power beam and measuring the broadband absorption with the simultaneously provided supercontinuum (probe). The third part of this thesis the laser-noise know-how to address a technological problem, namely the development custom, low-noise fiber lasers for LADAR applications. Two different fiber laser systems are constructed as transmitter sources of direct detection and coherent detection LADAR systems and tested for realistic scenarios. Both LADAR systems succeeded to detect 1 cm-diameter wire from a distance of 1 km in a measurement time shorter than 100 s, which is comparable to the best performing commercial LADAR systems.